Embodiments of the present invention relate generally to integrated circuits, and in particular to integrated circuits that include current mirrors.
Current mirrors are popular structures that exist in many electrical circuits. Some circuits use a current mirror to reproduce, or xe2x80x9cmirror,xe2x80x9d an input current to obtain an output current. In most cases, the output and input currents have the same value. In some cases, the output current is proportional to the input current.
FIG. 1 shows a conventional current mirror 100. Current mirror 100 has a transistor 112 connected to an input node 116 to receive an input current Iin. Transistor 112 connects to another transistor 114 at a reference node 118. Transistor 114 provides an output current Iout. The input node has an input voltage Vin. The reference node has a reference voltage Vref. Transistor 112 has its drain and gate connected together at both the input and reference nodes. Since the input and reference nodes connect together, Vin equals, Vref. Transistors 112 and 114 typically have the same construction such that they are matched. Hence, Iout flowing through transistor 112 equals Iin flowing through transistor 114.
In a typical circuit that includes current mirror 100, the value of Vin is insignificant in comparison to the supply voltage of the circuit. However, as the trend in reducing supply voltage of circuits becomes a necessity for some applications, the value of Vin becomes a significant issue. In some circuits with reduced supply voltage, Vin needs to be reduced to maintain a proper headroom voltage so that the circuits operate properly. However, because nodes 116 and 118 connect together, reducing Vin at node 116 also reduces Vref at node 118. In some applications, Vref needs to remain at certain value to drive transistor 414. Therefore, in some applications, as the supply voltage is reduced, current mirror 100 will operate improperly. For
For these and other reasons stated below, and which will become apparent to those skilled in the art upon reading and understanding the present specification, there is a need for improved current mirrors.